1. Field of the Invention
The present invention relates generally to digital image processing and, more particularly, to improved techniques for automatic exposure control when capturing digital images.
2. Description of the Background Art
Today, digital imaging, particularly in the form of digital cameras, is a prevalent reality that affords a new way to capture photos using a solid-state image sensor instead of traditional film. A digital camera functions by recording incoming light on some sort of sensing mechanism and then processes that information (basically, through analog-to-digital conversion) to create a memory image of the target picture. A digital camera's biggest advantage is that it creates images digitally thus making it easy to transfer images between all kinds of devices and applications. For instance, one can easily insert digital images into word processing documents, send them by e-mail to friends, or post them on a Web site where anyone in the world can see them. Additionally, one can use photo-editing software to manipulate digital images to improve or alter them. For example, one can crop them, remove red-eye, change colors or contrast, and even add and delete elements. Digital cameras also provide immediate access to one's images, thus avoiding the hassle and delay of film processing. All told, digital photography is becoming increasingly popular because of the flexibility it gives the user when he or she wants to use or distribute an image.
The defining difference between digital cameras and those of the film variety is the medium used to record the image. While a conventional camera uses film, digital cameras use an array of digital image sensors. When the shutter opens, rather than exposing film, the digital camera collects light on an image sensor, a solid-state electronic device. The image sensor contains a grid of tiny photosites that convert light shining on them to electrical charges. The image sensor may be of the charged-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) varieties. Most digital cameras employ charge-coupled device (CCD) image sensors, but newer cameras are using image sensors of the complimentary metal-oxide semiconductor (CMOS) variety. Also referred to by the acronym CIS (for CMOS image sensors), this newer type of sensor is less expensive than its CCD counterpart and requires less power.
During camera operation, an image is focused through the camera lens so that it will fall on the image sensor. Depending on a given image, varying amounts of light hit each photosite, resulting in varying amounts of electrical charge at the photosites. These charges can then be measured and converted into digital information that indicates how much light hit each site which, in turn, can be used to recreate the image. When the exposure is completed, the sensor is much like a checkerboard, with different numbers of checkers (electrons) piled on each square (photosite). When the image is read off of the sensor, the stored electrons are converted to a series of analog charges which are then converted to digital values by an Analog-to-Digital (A-to-D) converter, which indicates how much light hit each site which, in turn, can be used to recreate the image.
As with conventional (i.e., non-digital) cameras, the exposure (i.e., the amount of light that reaches the image sensor) determines how light or dark the resulting photograph will be. When the shutter opens, light (reflected from the subject and focused by the lens) strikes the image sensor inside the camera. If too much light strikes the image sensor, the photograph will be overexposed. Overexposure may result in a washed out and faded looking picture. Too little light, on the other hand, produces an underexposed photograph that may be dark and lacking in details, especially in shadow areas. In conventional cameras, the amount of light that exposes the image is controlled by adjusting either the aperture (the size of the opening through which light enters the camera) or the shutter speed (the length of time light is allowed to enter). These adjustments may be made manually by the camera user or may be made by an automatic exposure mechanism of the camera which makes one or both of these adjustments for the user. Automatic exposure control (or autoexposure) is a feature desired by many camera users, particularly those that are not photography experts, as automatic exposure control enables users to take photographs without having to manually adjust the lens aperture and shutter speed (exposure time). Moreover, because autoexposure is a feature that has been available for a number of years on conventional film cameras, many users expect autoexposure to be provided as a standard feature of any digital camera they may purchase.
Automatic exposure control systems of traditional film cameras typically use light sensors and analog circuits to determine the correct exposure settings for image capture. As described above, an automatic exposure control system in a conventional camera automatically adjusts the aperture and shutter speed settings based upon the amount of light detected by a light sensor. After the system measures brightness (i.e., how light or dark the scene is), it calculates and sets the aperture and shutter speed of the camera to render this level of light as “middle gray” (i.e., the middle of a grayscale spectrum from white to black) in the photograph. If the amount of light is high, the aperture setting and/or shutter speed (exposure time) will be reduced. In darker conditions, the system will adjust the aperture setting and/or exposure time to be larger. Current digital cameras perform a similar process digitally by recording light signals on a sensor and applying an algorithm to the signal to establish the exposure settings. The sensor used may be the digital camera's main CMOS (or CCD) sensor itself or, alternatively, a separate light sensor included as part of the digital camera.
Although most current digital cameras use a separate light sensor for automatic exposure control, there are drawbacks to this approach. One drawback is cost. Use of an external light sensor adds another component to the camera that increases the overall cost of manufacture of the camera. In addition, a light sensor may also increase size of the camera and its power consumption. Both of these factors can be significant considerations, particularly for smaller cameras where size and power requirements are critical design considerations. For these reasons, an autoexposure approach that does not require a separate light sensor is preferred.
Another problem is that existing autoexposure systems typically rely on the use of a flash device for lower light conditions. However, use of a flash device also adds to the cost of manufacture of the camera as well as increasing its size and its power requirements. One would think with present-day digital technology and scale, one could create a digital camera that is extremely small and portable, particularly since a digital camera is not constrained by the physical constraints of traditional photographic film. This is not the case today, however. As it turns out, the whole process of capturing light and generating a color digital image, such as with a digital camera, is a very compute-intensive process. The process of recording an image on photographic film, in comparison, relies on straightforward chemical reactions, all without the need for computing resources. A digital image, however, entails a process of converting light into electrical signals, converting those electrical signals into digital or binary information, arranging that information into a visual representation, applying various digital filters and/or transformations, interpolating color from that representation, and so forth and so on. The process of rendering a meaningful digital picture is a compute-intensive undertaking, roughly equivalent in processing power to that required today for a desktop workstation, yet done so within the confines of a handheld portable device.
The upshot of this substantial processing requirement is that, paradoxically, digital cameras today are relatively bulky devices since they require relatively large batteries to support their processing needs. This is easily seen today in camera designs. Many digital camera designs employ either large custom lithium batteries or, alternatively four to six AA batteries. Even with these large battery arrangements, digital cameras today have relatively short battery lives, such that the digital camera user is required to change out batteries at frequent intervals. Perhaps the biggest drawback of such an approach, however, is the added bulk imparted to the camera itself with such a design. Today, most of the weight of a digital camera is attributable to its batteries. Thus, present-day digital cameras, being constrained by their battery requirements, are generally no smaller or portable than their non-digital counterparts (e.g., standard 35 mm film camera). And the smallest cameras today still remain film-based cameras, not digital ones, due in large part to the battery constraints of digital cameras.
Because of these size and power consumption considerations, an improved approach for automatic exposure control is needed that does not require a separate light sensor or a flash device for low light conditions. An autoexposure mechanism that uses the digital camera's image sensor and does not require the camera to be equipped with a flash device is required in order to enable the production of smaller, less expensive digital cameras. Avoiding the manufacturing costs and power consumption requirements of including a flash and external light sensor on a digital camera enables production of smaller, more portable cameras that are less expensive and have reduced battery requirements.
Current autoexposure systems which rely on using only the main (e.g., CMOS) image sensor of a digital camera may, however, have problems in properly capturing an image in low light conditions without using a flash. Without flash, a longer exposure (processing) time is required for darker scenes in order to avoid having an underexposed picture. This longer exposure time may result in a blurred picture as the camera or the subject may move while the image is still being processed. In low light settings the processing time required without flash may be impractical using current digital camera autoexposure methods. One possible approach to this problem is to pre-compute an exposure solution by periodically or constantly evaluating the light conditions to provide for quicker processing of a correct exposure solution. A drawback to this approach is that these continual calculations may result in considerable power consumption, thereby requiring larger batteries and/or more frequent battery changes or recharging.
Another problem with current autoexposure systems is that these systems often rely on spot metering in which only a small area in the middle of the image is evaluated. Rather than evaluating an entire image, this approach evaluates only specific spots or areas, typically a small area in the middle area of the image shown in the viewfinder. Spot metering may be problematic if, for instance, a dark object is located off center against a very light background. In this instance, the dark object may not be exposed properly because it is not located in the area the meter is emphasizing. Because only a portion of the scene is examined, other regions that are significantly lighter or darker may be underexposed when a spot metering technique is utilized. For these reasons an approach that examines the entire image is preferred as it more reliably handles a wider range of situations.
What is required is an improved automatic exposure control solution that meters all regions of an image to reliably handle a wide range of conditions, from low light, indoor scenes to outdoor, bright sun settings. This solution should not require the digital camera to be equipped with a flash or a separate light sensor, enabling its use in a wide range of devices, including small, inexpensive digital cameras in which cost, size and power consumption requirements are critical. Ideally, the solution should also enable automatic exposure control in digital cameras with a fixed lens aperture. The present invention provides a solution that fulfills these and other needs.